1. Field of the Invention
Provided is a process for preparing alkyl aromatic compounds in a single reactor More specifically, an integrated process in a single reactor using a dual catalyst system is provided where alkanes are first dehydrogenated to create olefins, and then the olefins react with arene molecules.
2. Description of the Related Art
Alkyl aromatics are organic compounds comprised of an alkyl radical attached to an aromatic ring such as benzene. Alkyl-substituted aromatic compounds currently are prepared via a two-step process requiring separate streams of olefins and aromatics.
Given the large scale application of alkyl-aromatic derived detergents or surfactants, a number of routes have been developed to produce alkylbenzenes.
The HF/n-paraffins process involves dehydrogenation of n-paraffins to olefins, and subsequent reaction with benzene using hydrogen fluoride as the catalyst. This process accounts for the majority of the installed linear alkyl aromatic production in the world. It includes a stage where n-paraffins are converted to mono-olefins (typically internal mono-olefins), a unit whose primary function is to convert residual diolefins to mono-olefins, a unit which is essentially an aromatic removal unit—introduced before the alkylation step to improve yield and quality, and an alkylation step where mono-olefins, both internal and alpha olefins, are reacted with benzene to produce alkyl aromatics in the presence of an HF catalyst.
Another process involves dehydrogenation of n-paraffins to olefins, and subsequent reaction with benzene using a fixed bed catalyst. This is newer technology and has several of the stages depicted in the HF/n-paraffins process, but it is principally different in the benzene alkylation step, during which a solid-state catalyst is employed.
A Friedel-Crafts alkylation process involves chlorination of n-paraffins to monochloroparaffins followed by alkylation of benzene using aluminum chloride (AlCl3) catalyst. This method is one of the oldest commercial routes to alkyl aromatics.
Iridium complexes as catalysts are known. During the 1980s, it was discovered that certain iridium complexes are capable of catalytically dehydrogenating alkanes to alkenes under exceptionally mild thermal (i.e., less than 160° C.) or even photolytic conditions (see, e.g., J. Am. Chem. Soc. 104 (1982) 107; 109 (1987) 8025; J. Chem. Soc., Chem. Commun. (1985) 1829). For a more recent example, see Organometallics 15 (1996) 1532.
Pincer ligand complexes of rhodium and iridium as catalysts for the dehydrogenation of alkanes are receiving widespread attention. See, for example, F. Liu, E. Pak, B. Singh, C. M. Jensen and A. S. Goldman, “Dehydrogenation of n-Alkanes Catalyzed by Iridium “Pincer” Complexes: Regioselective Formation of α-olefins,” J. Am. Chem. Soc. 1999, 121, 4086-4087; F. Liu and A. S. Goldman, “Efficient thermochemical alkane dehydrogenation and isomerization catalyzed by an iridium pincer complex,” Chem. Comm. 1999, 655-656; and C. M. Jensen, “Iridium PCP pincer complexes: highly active and robust catalysts for novel homogenous aliphatic dehydrogenations,” Chem. Comm. 1999, 2443-2449. The use of compounds such as (PCP)MH2 (PCP=C6H3(CH2PBut2)2-2,6) (M=Rh, Ir) (1a, 1b) dehydrogenate various cycloalkanes to cycloalkenes at 200° C. with turnovers of 70-80 turnovers/hour. The reaction proceeds at 200° C. in neat solvent and without the use of a sacrificial hydrogen acceptor such as tert-butyl ethylene.
In addition, “pincer” complexes of platinum-group metals have been known since the late 1970s (see, e.g., J. Chem. Soc., Dalton Trans. (1976) 1020). Pincer complexes have a metal center and a pincer skeleton. The pincer skeleton is a tridentate ligand that generally coordinates with meridionial geometry. The use of pincer complexes in organic synthesis, including their use as low-temperature alkane dehydrogenation catalysts, was exploited during the 1990s and is the subject of two review articles (see Angew. Chem. Int. Ed. 40 (2001) 3751 and Tetrahedron 59 (2003)). See also U.S. Pat. No. 5,780,701. Jensen et al. (Chem. Commun. 1997 461) used iridium pincer complexes to dehydrogenate ethylbenzene to styrene at 150 to 200° C. Recently, pincer complexes have been developed that dehydrogenate hydrocarbons at even lower temperatures. For some recent examples, see J. Mol. Catal. A 189 (2002) 95, 111 and Chem. Commun. (1999) 2443.
Improvements in the selectivity and efficiency of preparing alkyl aromatics would be of great value to the industry. Limiting the number of steps needed would greatly enhance the efficiency of the process.